What is Responsive Web Design? How could it be used to enhance the UX and UI? Why isn't Facebook using RWD? Real, scalable RWD? I must be way off base, but

What is Responsive Web Design? How could it be used to enhance the UX and UI? Why isn't Facebook using RWD? Real, scalable RWD? I must be way off base, but their issues with the News Feed design are surmountable. Not impressed by Julie. I have never thought of FB as a bastion of design...and I have not yet been swayed to think or see otherwise.